When a vehicle travels over a road surface, the major mass of the vehicle is isolated, by its suspension, from vibrations caused by irregularities of that surface. The loads to which the vehicle suspension is subjected, are borne by the leaf springs of the vehicle suspension.
The softer the vehicle leaf springs, and/or the lower their load deflection rates are, the better the isolation.
The quality of this isolation affects the ride quality of the vehicle and any damage inflicted on the road surface thereby.
There is, however, a limitation on the degree to which the leaf springs can be softened, because the suspensions also have to control the dynamic forces exerted upon them by the mass of the vehicle during changes of direction and/or velocity thereof.
One such direction change occurs when a vehicle changes its direction of travel when being driven around, say, a curve or bend in the road.
During this manoeuvre, the vehicle suspension has to accommodate the centrifugal forces which cause the mass of the vehicle to transfer on to the wheels on the outside of the curve or bend and from the wheels on the inside of the curve or bend.
This mass transfer on to the outside wheels of the vehicle is transmitted to the suspension. The softer the suspension, the more the leaf springs will deflect, thus causing the vehicle to lean or roll.
Owing to the practicalities associated with the design and installation of such a suspension, as well as other dynamic vehicle handling characteristics, there is a limit to the amount and rate of roll which is acceptable. Such a limitation creates a compromise between the ride and handling qualities of the vehicle.
With such a compromise, the amount and rate of roll which is acceptable, limits the softness of the suspension and the associated quality of ride which is available.
To improve or resolve this compromise, additional springs or spring modifications, have been available, which resist vehicle roll, without increasing the vertical spring stiffness, when both vehicle wheels deflect together.
Such mechanisms are usually known as “anti-roll mechanisms”.
However, as the vehicle ride is also dependent upon the vibrations to which the suspension is subjected when only one wheel of the vehicle deflects, there is often a limit as to how much the ride and handling compromise can be extended.
For many years, anti-roll mechanisms have been based upon a separate torsion bar or tube acting transversely of the vehicle between the opposed wheels of the suspension.
A recent development for vehicles which are suspended by leaf springs on each side thereof, have been mechanisms which stiffen the springs internally, only when the springs deflect in opposite directions, as they do when the vehicle rolls.
Leaf springs to which a vehicle chassis is mounted, are effectively pin-jointed beams and set as such. Usually, the leaf springs are fastened to the axle of the wheels in the region of their centres and are mounted to the vehicle chassis via bushes and/or shackles at their respective opposed ends.
The loads applied to the leaf springs create bending moments therein, which, in turn, cause the springs to deflect and, thus, absorb energy.
During vehicle roll, the effective mass of the vehicle is transferred, at the axle, from one spring to the other, changing the bending moments therein. The stiffness of the leaf springs controls the change in deflection of each spring and these now different deflections in each spring, on each side of the vehicle, create, in turn, the amount of roll in the vehicle.
In previous developments of these anti-roll techniques, a torsionally rigid member has been connected between the leaf springs at or adjacent one end of the springs. This member, such as an anti-roll bar or tube, allows the leaf springs to work normally when they deflect together in the same direction, as they normally function when creating the vehicle's primary ride characteristics.
As discussed above, when the vehicle rolls, the leaf springs deflect in different directions as the vehicle mass is transferred to the outside spring from the inside spring. During such deflections, the torsionally rigid member resists the angular differences between the two opposed leaf springs, thereby creating a deflection resisting moment in the springs, which then produces a lower change in bending moment in the springs. This, in effect, changes the pin-jointed beam nature of the springs into fixed-ended, or encastre, beams.
This lower-than-previous change in bending moment creates smaller spring deflections and thus stiffens the springs during roll only.
Therefore, adding the torsionally stiff member to the ends of the leaf springs, creates an effective anti-roll mechanism.
In practice, this additional bending moment is applied over the physical length of the brackets mounting the torsionally rigid member to the ends of the opposed leaf springs.
Also, it can be seen that this anti-roll mechanism reduces the maximum, and any change in the, bending moment and, therefore, increases the service life of the leaf springs.
In practice also, this mechanism is more effective as an anti-roll mechanism than suggested above. If the torsionally rigid member is added to just one end of the pair of opposed leaf springs, the resultant anti-roll mechanism stiffens just one end of the leaf springs when the vehicle rolls. This creates asymmetrical cantilever deflecting leaf springs, which means that, during vehicle roll, the axle seat area of each leaf spring, which is fastened centrally to the axle, attempts to deflect in different angular directions. Such deflection is resisted by the torsional rigidity of the axle which, again, tends to stiffen-up the leaf springs. This anti-roll stiffness associated with asymmetrical springs is well known in spring and suspension design practice.
Whilst this anti-roll stiffening of the leaf springs can be very effective, some applications can benefit from even higher and extra spring stiffening for resisting vehicle roll. This could allow the normal ride stiffness to be lowered even further, thus improving the basic vehicle ride and creating an even better compromise between ride quality and anti-roll vehicle handling stability.
To summarise current prior art anti-roll suspension designs, it is normal practise to mount the torsionally rigid member as close as possible to the neutral axis in bending of the leaf springs.
Also, the mountings between the torsionally rigid member and the opposed leaf springs are made sufficiently flexible to allow for the transmission of torque created by the member but to be considered pin-jointed in plan view.
This is achieved by making the brackets mounting the opposed ends of the torsionally rigid member to the ends of the leaf springs, sufficiently narrow or otherwise flexible, to allow the member to move within those mountings.
This is considered to be normal practice to improve assembly, reduce local stresses, create a lightweight structure and avoid interfering with the normal spring deflections.
It is an object of the present invention to provide a leaf spring suspension which provides, during roll of the associated vehicle, further stiffening of the springs beyond that achieved to date, as discussed above, to improve the anti-roll characteristic of the suspension and, as a consequence, its performance and durability.